There are hundreds of types of capacitance sensors in use today. These sensors can be extremely sensitive and stable devices. Their operation is based on changes on the sensor's capacitance (c) according to formula 1, below:
                    c        =                              ɛ            0                    ⁢                      ɛ            r                    ⁢                      A            d                                              [        1        ]            
Such capacitance changes are induced by the quantity to be sensed. In our case, that is the applied strain. In the above expression, ∈0 is the free space permittivity (8.854×10−12 C2/Nm2), ∈r is the relative permittivity of the dielectric material between the electrodes, A is the surface area in m2 and d is the distance between the electrodes in m. By varying the dielectric properties of the material or the geometry of the electrodes during the applied strain, transference of the charge on the electrodes can be measured and consequently the strain can be detected.
Traditional capacitance strain sensors are made of two parallel plates with soft dielectric material in between. Their sensing capabilities are based on detecting variations in the separation or the overlap between electrodes. An applied strain results in a movement on the electrode and a mechanical-electrical energy transference causing a change on the capacitance. In order to further magnify the strain displacement, some of these devices employ long cantilever beams or membranes. With the development of microfabrication and MEMS techniques, complex systems with device sizes in the micro- or nano-meter scale can be constructed. The small size of these devices can help to realize high resolution, wireless measurements and miniaturization of the entire systems.
Lately, interdigital capacitors—planar structures of flexible material with no physical displacement of electrodes—are being used to detect deformation. The sensing principle is based on the deformation of the electrodes and spaces between them. However, the existing strain sensors do not have passive wireless function.